Optical fiber amplifiers have found increasing use in optical transmission systems. As known, the fiber is doped with a rare earth element such as Erbium, and a pump signal with an appropriate wavelength is applied to the fiber to excite ions in the fiber. When a message signal is also applied, it is amplified by the excited ions. The fiber amplifier also usually includes at least one optical isolator to prevent unwanted reflections.
Increasingly, the need exists for use of the amplifier with polarized light. This means that the amplifier should preferably be able to amplify incoming light which is polarized in either or both orthogonal directions of polarization as well as light which is unpolarized. Some proposals have been made to include polarization maintaining fiber and polarization dependent isolators in the fiber amplifier. (See, e.g., Sakai, et al "Polarization-Maintaining Erbium-Doped Fiber Amplifier" (OAA Proceedings, Jul. 11-13, 1996) pp. 170-173, U.S. Pat. No. 5,303,314 issued to Duling et al, and Japanese Patent Application No. HEI 7(1995)-142798.) Generally, such amplifiers have been adapted to receive signal light having a polarization in only one direction.
A further concern in dealing with polarized message and pump light is that of "hole burning" which is the depletion of ions in the fiber and can be caused by the pump light exciting ions in a direction which is different from the direction of polarization of the message signal. Consequently, it is also desirable to ensure that the direction of polarization of the pump and message signal is the same in the fiber amplifier.